GRANTS PASS, Ore. -- Scientists taking a new look at old videotapes of the muddy seafloors off southern Oregon found that places showing tracks from the nets of fishing trawlers had fewer numbers and kinds of fish than areas that were undisturbed.

Other studies worldwide have documented the damage bottom trawling does to seafloor habitats, but this is the first to look at fish numbers and diversity on muddy seafloors on the West Coast's Continental Shelf, where bottom trawlers do much of their work, the study authors said.

A review of videotapes taken in 1990 from a manned submersible in an area known as the Coquille Bank off southern Oregon found that in areas showing roller tracks in the mud from bottom trawling nets, there were 20 percent fewer fish, 30 percent fewer species of fish and six times fewer invertebrates, such as crabs and seapens.

"We are not suggesting trawling be banned," said Mark Hixon, a professor of marine biology at Oregon State University and lead author of the study published in the Journal of Experimental Marine Biology and Ecology.

"The question must be asked whether we want to sacrifice these ecological communities, not even knowing what the long-term effects of bottom trawling might be, or whether some mud areas of the Continental Shelf deserve permanent protection," he said.

The areas disturbed by fishing tended to have more scavengers, such as sea stars, hermit crabs and hagfish, which may have been attracted by burrowing organisms exposed by the trawling gear, Hixon added.

Two years ago, federal fisheries managers banned bottom trawling on 300,000 square miles off the West Coast to protect coral beds, kelp forests, rocky reefs and other areas deemed essential fish habitat. But there have been no efforts to protect the muddy seafloor that covers most of the Continental Shelf, scientists said. Some areas of the shelf are temporarily off-limits to bottom trawling until rockfish populations rebuild from overfishing.

"There could be other possible explanations for the patterns they see, but the most likely explanation would be the differences they observed from trawled areas to untrawled areas," said Andrew Rosenberg, professor of natural resources at the University of New Hampshire and a former deputy director of NOAA Fisheries, the federal agency that oversees ocean fishing.

"This doesn't mean you shouldn't do trawling everywhere," said Rosenberg, who did not take part in the study. "You have to manage where trawling occurs and what level of impact we can sustain without reducing the resource productivity. This kind of study provides some good information that allows you to start to do that."

He added that muddy seafloors can be very productive ecosystems, and fish habitats that have been damaged by fishing may have a harder time recovering as global warming changes ocean conditions, making it prudent to start thinking about protecting more areas.

About 120 bottom trawlers are still fishing off the West Coast, down from about 500 in 15 years, said Brad Pettinger, administrator for the Oregon Trawl Commission. They catch shrimp, sole, rockfish and other bottom-dwelling species popular in West Coast fish markets, as well as hagfish, black cod, and thornyheads that are exported to Asia.

Pettinger said fishing has never been better, and there is little of the muddy seafloor on the shelf that hasn't been fished. He doubted that areas scientists identified as undisturbed actually were, because currents would quickly cover the tracks left by fishing gear.

Hixon, who was in the submersible that made the videotapes in 1990 as part of a survey for likely places to drill for oil and gas, said muddy ocean bottom at depths of 600 feet to 1,200 feet is actually very still, undisturbed by surface waves and currents, leading him to believe trawl tracks would last a long time.

Susan Murray, Pacific director of the conservation group Oceana, said the study clearly showed new ways of fishing need to be developed that don't harm marine habitats.